Brazing methods performed by a continuous furnace have been well known. Typically a plurality of metallic parts to be brazed together are brought in abutment with a metallic solder having a melting point lower than the metallic parts and deposited between portions of the metallic parts to be joined, and they are passed through the continuous furnace so that the metallic parts are not melted but the metallic solder is melted whereby the spaces between the portions are filled up with the solder thus melted and moved by a capillary action into such spaces, and then the metallic parts with the solder are cooled.
In such brazing methods, it is generally not preferable to have the metallic parts subjected for a long period of time to such high temperature at which the metallic solder melts. In other words, it is desirable to subject them under such high temperature for as short a time as possible.
For example, when steel parts are brazed by a copper solder, the solder would excessively penetrate into iron crystalline spaces of the steel parts or produce weak alloys with iron elements, resulting in making the mechanical strength of joint portions low, if the brazing at a higher temperature is continued for a long period of time and the solder is accordingly kept liquid for the long period of time.
Also in case of brazing aluminum alloy parts which are joined generally by means of a solder made of Al-Si alloys, said solder would make the aluminum portion of the aluminum alloy parts eutectic and decompose aluminum alloy structures of the parts, if the solder is melted and kept liquid for a long period of time.
Such drawbacks as mentioned above could be avoided if temperatures of metallic parts to be brazed are raised evenly as a whole, for example by inserting all of the parts at one time in a batch-type furnace, and they are kept at a brazing high temperature only for a minimum period of time. However, this way of heating the parts in continuous brazing furnaces is very difficult, because, they have not the same dimensional volume as batch-type furnaces, and their thermal characteristics are different.
For example as shown in FIG. 1, when an aluminum part 1 having a thickness of 5 mm and another aluminum part 2 having a thickness of 1 mm are heated together in a furnace, the thin plate 1 reaches first, as a natural consequence, a high temperature, while the thick plate 2 reaches belatedly said high temperature.
This means that if a heating curve is selected so as to be proper for heating the thin part 1, a brazing solder on the thick portion 2 will not be melted during the time T shown in FIG. 1, because the thick portion 2 will not have been heated to the desired brazing temperature until near the end of the time interval T. And, on the contrary, if the heating curve is selected so as to be suitable for heating the thick part, the thin part will overheated adversely affecting its physical properties.
In other words, if during preheating the parts exhibit a difference of temperatures which is represented by d in FIG. 1, it is necessary to have them subjected to a high brazing temperature for a comparatively long period of time expressed by T in FIG. 1 in order to make sure that the part 2 will reach the brazing temperature necessary for melting the brazing solder.